1) Field of the Invention
The present invention relates to an exchange apparatus and a method for testing an exchange apparatus.
2) Description of the Related Art
Generally, the ISDN (integrated Services Digital Network) subscriber interface consists of a B-channel for transferring user information and a D channel for transferring a line exchange signal to control the B-channel. For example, in the case of a 1.5 Mbps primary rate interface subscriber, the ISDN subscriber interface is formed of 23 B-channels and a D channel.
The 23 B-channels are controlled by exchanging a call process signal called a layer 3 message between an office exchange and a primary rate interface subscriber in the D-channel. In order to pass the layer 3 message through the D-channel, it is essential that the logical links of a layer 1 and a layer 2, each being lower than that of the layer 3, are in an established state.
The logical link of the layer 2 is formed by connecting a primary rate interface subscriber with an office exchange by means of hardware, synchronizing with the electrical signal frame of the layer 1, and then implementing the logical link establishing procedure shown in FIG. 12. That is, a setting request "SABME (set asynchronous balanced mode extended)" message in a multiframe mode is first transmitted from the office exchange to a primary rate interface subscriber (step A1). Then the primary rate interface subscriber received the setting request "SABME" sends back a "UA (unnumbered acknowledge)" message as an acknowledge to "SABME" to the office exchange (step A2).
In other words, when an office exchange can receive an acknowledge "UA" message to the "SABME" message, a layer 2 logical link can be established between the office exchange and the primary rate interface subscriber.
The layer 2 logical link which has been once established between the office exchange and the primary rate interface subscriber is regulated so as to maintain always its establishment state. The establishment is confirmed by transmitting and receiving a monitor frame between the office exchange and the primary rate interface subscriber.
The "SABME" message has the data configuration shown in FIG. 13. The "UA" message has the data configuration shown in FIG. 14. In FIGS. 13 and 14, the SAPI (service access point identifier) of the octet 2 is data representing the type of the service point provided by the layer 2. For example, "0" defines a call control signal service point of a layer 3. "16" defines a packet data service point.
C/R (command/response) bit is data which is defined as a command when a special request is requested to a frame receiving side or as a response when an acknowledgment is sent to the special request. The C/R bit, for example, is shown in FIG. 15.
That is, since the "SABME" message is a logical link setting request (command) sent from an office exchange to a primary rate interface subscriber, the C/R bit is set to "1". message is an acknowledgment (response) sent from a primary rate interface subscriber to an office exchange, the C/R bit is set to "1". This definition accords with the standard convention ruled by ITU-T.
"EA" of the octet 2 is data called an address field extension bit. For example, "0" means that the data follows a next octet and "1" means that the octet is the final octet.
"TEI (terminal endpoint identifier)" of the octet 3 represents a terminal identifier. "P/F (poll/final)" bit of the octet 4 is data used as a poll bit in the case of command and as a final bit in the case of response. Hence, in the case where the "SABME" message is a command, the data represents a poll bit. As to the P/F bit, "1" is set similarly to the final bit of a response to a command of "poll bit=1".
In the octets 5 and 6, "FCS (flame check sequence)" is data for detecting data transmission errors.
After the layer 2 logical link becomes an establishment state as a result of an exchange of the "SABME" message and the "UA" message, transmitting and receiving the layer 3 message between the office exchange and the primary rate interface allows the B-channel signal path for transfering user information such as voice to be established or released.
By referring to FIG. 16, explanation will be made here as to the layer 3 message transmission and reception including, for example, a normal process ranging from an establishment of a voice signal path in the B-channel to a release.
First, when a primary rate interface subscriber on the originating side originates a call, SETUP (call setting request) message is sent to a primary rate interface subscriber on the designation side (step B1). Then CALLPROC (call proceeding: call setting) message is transmitted from the office exchange to the primary rate interface subscriber on the originating side (step B2).
Thereafter, when calling the primary rate interface subscriber on the designation side starts, ALERT (alerting: called party calling) message is transmitted to the primary rate interface subscriber through the office exchange (step B3). When the destination side responds to the calling, CONN (connect: called party's response) message is transmitted from the destination side to the originating side through the office exchange (step B4) so that a channel (in this case, the B-channel) is connected to begin communications. At this time, the office exchange is sending the CONNACK (connect acknowledge) message representing a complete connection between the originating side and the destination side to the destination side (step B5).
Next, at the end of the communications, a DISC (disconnect: call disconnection request) is transmitted from the destination side to the originating side through the office exchange (step B6) so that a channel disconnection and a call number are released. Then a REL (release: channel disconnection completion and a call number release request) message is transmitted from the originating side to the destination side (step B7). When the REL message is received on the destination side, RELCOM (release complete: channel release completion report and call number release request) message representing the release completion is transmitted to the originating side (step B8).
At this time, a selection of the B-channel used is decided by transmitting and receiving the layer 3 message. On the office exchange side, B-channels are searched for an empty channel from lower or higher number in order. Then an empty B-channel (not being used) number first found is designated.
In the general B-channel test of a primary rate interface subscriber, as shown in FIG. 17, a loop-back connection of the office exchange 20A and primary rate interfaces 30A and 40A is made by a switching operation at the primary rate interfaces (digital terminals) 30A and 40A each called a digital terminal (DT). The layer 1 is first established by judging the usable state of the hardware B-channel, based on the defect state of a signal returned to the office exchange due to electrical current flowing through the loop-back connection.
Thereafter, communication confirmation is made between the telephone terminals 7A-1 to 7A-N and telephone terminals 8A-1 to 8A-N (N is a natural number), by actually connecting the primary rate interface subscriber devices 50A and 60A to each other. Thus the logical links of the layer 2 and the layer 3 are established to perform the B-channel test of the primary rate interface subscriber.
Instead of telephone terminals 7A-1 to 7A-N and 8A-1 to 8A-N shown in FIG. 17, Japanese Patent Application Laid-Open (Kokai) Patent No. HEI 4-51641 discloses a technology in which testing is made by preparing two conventional analog dummy call devices and then by making all B-channels over the primary rate interface subscriber in use state to obtain a heavy load.
However, such a general testing method has a disadvantage in that if the primary rate interface subscriber (hereinafter sometimes merely referred to as a subscriber) is not actually connected after an establishment of the layer 1, a comprehensive call processing test including the layers 2 and 3 of the primary rate interface cannot be performed so that expensive primary rate interface subscriber devices must be prepared by the number of subscribers when plural primary rate interface subscribers want simultaneously a call process test.
Furthermore, it is general that a new function is being developed even on the primary rate interface subscriber side during a development of a new function on the office exchange side. Hence, there is a disadvantage in that it is difficult that a subscriber is actually connected to execute a call process test.
Where an operation begins by connecting newly a primary rate interface subscriber to an exchange, if an abnormal communication state should occur, in spite of a normal hardware connection (an establishment of layer 1), it is difficult to specify which of the primary rate interface subscriber side and office exchange side causes the abnormal state. Hence, there is a disadvantage in that much time consumed for an abnormal cause analysis leads to a poor service.
Moreover, according to the testing method described in the Japanese Patent Application Laid-open (Kokai) No. HEI 4-51641, a load test must be performed to all B-channels of primary rate interface subscribers. Hence, there is a disadvantage in that a specific B-channel selected cannot be tested.